Hi all who have this strange error.
I been fighting with this error and have tried a lot of solutions. The one who works with my installation is partly with help from "quest4denali".
Under Host->Advanced settings->Compression settings->Excluded Mime Types i added the 2 mimeType lines with the word 'application' and 'application/pdf'.
USE THE UPDATE BUTTON DIRECTLY UNDER THE "Whitespace filter:" INPUT FIELD.
If you use the Update button in the bottom of the page there will NOT be anything written to the "compression.config" file.
The changed file is the "compression.config" file in the root, not the file in the /config directory. (Check it!)

Now you can set the "Use Whitespace filter" On or Off and choose "Compression setting:" to No compression / Reduce compression (?) / GZip compression.
This time you must use the Update button in buttom of the page.

The download of files should work with all combinations.

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Niels-Jørgen Hvidberg http://newcomtech.dk http://njhvidberg.dk